The invention relates generally to power amplifiers, and in particular to linearizing the input/output transfer function for an amplifier, a particularly a high power Class AB power amplifier.
High power, broad band power amplifiers are well known. These amplifiers may operate in a feed-forward configuration, or may have other forms of linearization which are required when the main power amplifier operates, for example, as a Class AB amplifier. Although Class A amplifiers usually produce less distortion than Class AB amplifiers, Class A amplifiers are also less efficient than Class AB amplifiers. Thus, in order to retain the advantages of efficiency, while minimizing distortion, Class AB amplifier configurations have been developed which implement various forms of error or distortion correction.
One form of error correction is feed-forward correction which can use an injected pilot signal to correct distortions in the input signal caused by the Class AB amplifier. In another error correction approach, a predistortion circuit using, for example, a gain-phase circuit, can be provided with various adjustments to produce a gain-phase signal from the original signal, so that when the gain-phase signal is input to the power amplifier, operating as a Class AB amplifier, the output is a corrected amplification of the original input signal to the amplifier arrangement.
Even in a properly adjusted amplifier arrangement, using predistortion, a certain amount of instability, that is, drift in the operating point gain and/or phase, can be observed. As a result, a second loop, using an error amplifier, is employed and is tuned using, for example, the pilot signal noted above. While these remaining distortions can be attended to in the feed-forward cancellation loop circuitry, for example, the quadrature pilot signal detection and cancellation circuitry, such as that described in U.S. Pat. No. 5,796,304, the circuit cost is somewhat expensive.
The invention provides an advantageous approach toward maintaining an adequately linear input/output relationship in a distorting amplifier arrangement using a low cost approach employing a pilot signal.